The invention relates to microelectromechanical systems and microfabrication.
Photolithography is an iterative process used to transfer patterns from a photomask design onto a photosensitive material or photoresist. Once transferred into the photoresist, the pattern is then developed and “windows” to the underlying material are opened up. The underlying materials are then etched away, leaving behind a permanent pattern in the lower material, or materials are deposited into the windows. When fabricating surface micromachined microelectromechanical systems (MEMS) devices, thin film depositions are interlaced with photolithography, etching, and lift-off processing steps. Typically, positive photomasks and photoresists are used in MEMS fabrication to ensure fine resolution and precise minimum feature sizes. Sometimes, however, reverse field or complementary mechanical structures must be etched into the substrate (e.g., thermal isolation), which necessitates using a combination of surface and bulk micromachining and involving aggressive etch chemistries, negative photoresist, and robust masking materials. When these situations arise, the ability to use a positive photomask with a negative resist is often helpful to avoid fabrication delays and higher costs. In addition, certain negative photoresists (i.e., MicroChem's Nano™ SU-8) are desirable because when they are hard baked they become chemically and thermally resistant. This allows them to stand up very well to the aggressive etching profiles needed for bulk micromachining, such as SF6, when isotropic silicon etching is required.
AZ 5214E is a positive photoresist that also has the ability to be utilized for image reversal, providing the negative pattern of a photomask. It is most commonly utilized in the IR mode for lift-off processes where thin film metals are deposited and selectively patterned using a lift-off technique. Through an initial exposure, post-exposure bake (which acts as an image reversal bake), and flood exposure steps, image reversal is easily obtained. After a layer of AZ 5214E is exposed with a positive photomask, it is baked at a temperature between 115° C. and 125° C., which initiates an agent in the photoresist that crosslinks the areas that have been exposed. This step is critical, since if it is not baked at the right temperature (±1° C.) the negative pattern of the photomask will not be obtained and the AZ 5214E will just act like a positive photoresist. Therefore, it is important to determine and optimize the reversal bake temperature in the range mentioned above for individual processes. The exposure of a photoactive compound within the photoresist, and the crosslinking in the exposed areas, makes these exposed areas insoluble in the developer. Meanwhile, after a flood exposure, the unexposed areas develop away in a standard positive photoresist developer. Unfortunately, AZ 5214E alone is not robust enough to stand up to the aggressive bulk micromachining etching profiles.
SU-8 is a thick, epoxy-based, high contrast photoresist that is typically used in applications where it will be a permanent layer. Through an exposure and post-exposure bake steps, an SU-8 layer crosslinks and becomes resistant to liquid developers, and a wide range of other removal methods [e.g., O2 plasma ashing, reactive ion etching (RIE), corrosive etches, etc.], thus making SU-8 an excellent masking material for bulk micromachining. The exposure step creates an acid and the post-exposure bake step follows this up by thermally activating the acid to crosslink the exposed areas. The challenge here is that the SU-8 must remain uncrosslinked, and therefore unexposed, through a majority of the process. If the SU-8 gets exposed at any point it will crosslink and no longer be usable for this process. Various methods have been used to protect an uncrosslinked SU-8 layer during subsequent lithography and/or metal deposition steps, such as a filament evaporated metal layer, using an antireflective coating on top of the SU-8, UV exposure dose control, contact printing of a metal layer, and using a positive photoresist as a protection layer. These approaches have been used in applications where SU-8 is utilized as a sacrificial material, in the creation of microfluidic channels and other stacked structures, and for electroforming.